The Deep

[Bazile]

I watched a programme recently by David Attenborough , where he ventured deep below the ocean , in a special submersible craft , in order to film the weird and wonderful creatures that live miles below the ocean waves .

My question is - how do these creatures ( some extremely delicate looking ) survive the enormous pressure of the water at those extremes depths ?

[mibn2cweus]

This may be where life on Earth originated

[Bazile]

Thanks mibn - but what I am curious to know is , how they can survive at such enormous water pressures

[gen2]

If the body of these organisms is composed largely of water, then the internal pressure will equal the external pressure. There is nothing to crush so what is the problem? It is only if a chamber of air is taken to those depths, then it is liable to be crushed. The opposite is also true - if an organism at great depth is brought to the surface, then any dissolved gases in the liquids within the organism's body, will come out of solution and expand causing it to bloat, explode or most certainly die.

[theprof]

As an ex-diver myself, I'd like to put the pressure/depth relationship into perspective first, for you to see the enormous pressure that deep-sea creatures have to withstand.

The general rule is that there is an increase in pressure of one atmosphere for every ten metres of depth in the sea. At sea-level, the pressure is one atmosphere, ten metres below sea level it is two atmospheres and at one thousand metres, the pressure is over one hundred atmospheres.

Now the deepest part of the ocean is the Mariana Trench, which is off the coast of Guam. The bottom of the Mariana Trench is about 11000 metres below sea-level, which is somewhere around seven miles without getting my calculator out. In the early sixties, a submersible craft reached the bottom of the trench and discovered numerous examples of a species of flat-fish along with other creatures at that extraordinary depth.

(continued)

[theprof]

The reason why deep-sea creatures are able to withstand these enormous pressures is down to the structure of their bodies.

In humans, the body has large spaces, which do not contain dense tissue or organs. The air in these spaces expands at increasing depth and if we greatly exceeded the scuba diving depth of some seventy metres, we would end up looking like the Michelin Man just before we exploded. The only means of access to great depths like the Mariana Trench for humans is via pressurised submersible vehicles.

Now, many of the creatures that live at this depth, possess bodies that are completely filled with water. Under these circumstances, external pressure on the body has hardly any effect at all.

The reason this occurs is that the pressure in the internal tissues of the creature is the same as pressure being applied to the creature by the sea. Under these circumstances, the pressure difference is nil or at least, very close to it. For these creatures to be crushed, the pressure on the outside would have to differ considerably from that of the internal tissues. To put it another way, to be crushed, there needs to be a pressure difference.

Going back to the fish in the Mariana trench, as you probably know, many fish contain swim-bladders that enable them to rise and drop in the sea water column as the need arise. However, deep-sea fish do not have swim-bladders, but they do have glands that release gas internally into their bodies at the same pressure as the surrounding water. As the pressure difference is virtually zero between the interior and exterior of the fish, they are able to survive.

(continued)

[theprof]

Other creatures, such as squid found at great depths survive because they are virtually completely filled with water.

Sometimes deep-sea fish and other creatures are caught during specialised trawls at very great depth in the world. They are brought relatively rapidly to the surface and are unable to equalise their internal pressure with that at sea-level. They rapidly expand due to the pressure difference, which often results in their eyes popping out of the sockets and sometimes the fish literally explodes. Believe me, it�s not a pretty sight being splattered with fish as I�ve witnessed it first-hand myself.

Finally, there are differences in the biochemistry of deep-sea creatures compared to other animals which favour their survival at depth. These include a specific type of protein make-up which is exclusive to deep-sea creatures. I won't go into it here as it's a complex subject and usually needs a very good understanding of biochemistry.

[theprof]

OMG, I appear to have had a senior moment.

In the second paragraph of my second post, I should have said that the human body would be crushed at great depths rather than explode; the air spaces contract under external pressure.

[brachiopod]

I was a bit surprised to see that coming from you, prof !!

Started celebrating New Year early ??

;o)

[theprof]

Arggh, my secrets out. I did have a skinful Saturday night brachiopod. The shame of it - I'm on the wagon from now on!

[mibn2cweus]

theprof, There's one resolution I managed to break before I even had a chance to make it! Cheers to your good health . . . glub . . . glub . . . glub . . . D'oh! . . .
and may you never drink from the wrong beaker!